Which means either climate change has messed with Greek weather in the last 2,500 years or the makers of the “300” movies are taking artistic license: As befits a graphic-novel adaptation heavy on blood-spilling and war-mongering, the computer-generated skies of this world are perpetually roiling with black clouds and gloomy foreboding.

“300: Rise of an Empire” is based on Frank Miller’s “Xerxes,” and provides a sequel to the 2006 hit “300.” Or not quite a sequel, exactly; this is a rare instance in serial movie-making in which the action actually takes place at the same time as the events of the first film.

“300” was devoted to the battle of Thermopylae, the fabled 480 BC fracas in which a small contingent of Spartan warriors sacrificed themselves to hold back the invading Persian army.

“Rise of an Empire” shifts the action to sea, where the Athenian general Themistocles (Sullivan Stapleton) leads his ships into battle against the Greek turncoat Artemisia (Eva Green, from “Casino Royale”), who has allied herself with the Persians.

Many bone-crushing battles ensue. Director Noam Murro, a TV-commercial veteran, apes the style of the first “300,” with its geysers of digital blood splashing across the lens and its hiccuping slow-motion — sometimes in mid-decapitation, to savor the effect.

These are tiresome, and one waits impatiently for Eva Green’s imperious, kohl-eyed she-devil to stride into the scene and devour men whole. She doesn’t literally do that, of course, but the effect is the same.

There’s also Lena Headey, returning from the first movie and as full of bravado as before (we see a couple of glimpses of her character’s hubby, played by Gerard Butler, in old footage).

In fact, despite the overwhelming (and perhaps overcompensating) masculinity that dominates these films, the appeal here is almost entirely thanks to the two women.

The promise of a showdown between them is sadly unfulfilled, and would of course be historically inaccurate, if you’re still clinging to such old-timey notions.

Meanwhile, the array of six-pack abs that brought the first film a certain level of pop-culture notoriety is still in place, pumped up for 3D in some theaters.

The brave few at Thermopylae did not die in vain.

“300: Rise of an Empire” (one and a half stars)

The sequel to “300” happens at the same time as that brawny account of the battle of Thermopylae, but this time we watch a clash at sea. The movie’s fun whenever Eva Green’s witchy naval genius is around, but the rest is tiresome computer-generated blood-spilling.